Three radar antennas or ports, employing three simultaneously synthetic aperture arrays, have been used for clutter suppression interferometry. Each port is motion compensated for each radar transmitted pulse. The motion of the radar platform is matched to the antenna spacings in a manner such that, at the occurrence of each radar-transmitted pulse, the antenna moves exactly one antenna spacing. Therefore, in a three-port system, the last two antenna ports receive radar return data at the previous antenna position for three successive transmitted pulses. Phase compensation may be employed if positions are not exactly equal to the antenna spacing. Assuming exact spacing for three successively transmitted pulses, each antenna port occupies the same position in space; and three radar clutter returns from the ground will have the same amplitude and phase, but objects moving on the ground (movers) will have different positions and consequently will customarily have a different amplitude and phase.
In a prior art three-port system, data from each antenna port is spectrally processed utilizing a well-known Fast Fourier Transform (FFT). During a particular time interval there are "N" transmitted pulses and "M" range bins. In each range bin the "N" returns are spectrally processed in each antenna port. In each range bin of each port there are "N" Doppler bin outputs. The Doppler bin outputs correspond to the Doppler signals detected at each antenna port. The outputs from antenna ports 1 and 2 are subtracted so that in a particular range bin the corresponding Doppler bin output of port 2 is subtracted from that of port 1. If there are no movers, but just clutter, the subtraction will theoretically equal zero signifying no mover present. By subtracting the output signal vectors from the three ports, a phase angle is obtained which is proportional to the relative radial velocity of a target (mover) and is determinative of target true azimuth position.
In a simplified system of the prior art, three ports (antennas) may be considered as being spaced along a platform on a moving vehicle, such as an aircraft. Each of the ports radiates electromagnetic energy and, as the three ports move relative to a target, such as a ground based target, the second and third ports will see identical ground clutter. However, if there is a moving target on the ground (mover), there will be a displacement of the target when the second port arrives at the physical position that the first port occupied an instant before. This displacement also occurs for the detected signal by the third port when it moves to the position occupied by the second port an instant before. The displacement of such a mover results in phase shift of processed signals at the individual ports, which corresponds to relative radial velocity and the true target azimuth position, as will be now discussed in connection with the figures.
FIG. 1A-FIG. 5 indicate the three-port vector diagram technique for determining relative radial velocity of a mover. These diagrams, for purposes of illustration, have been simplified in a number of aspects to show the theory of operation.
What is shown is only four transmitted pulses per antenna port, instead of the hundreds that would actually be processed in a practical system. The spectrally processed signals from the four returns are vectorally added in a filter that is matched to an equivalent frequency. Because the antenna motion is compensated for in each port, the clutter in a particular Doppler bin is the same in all ports. Thus, by way of example, if the signal at the second port were subtracted from that at the first port, or if the signal at the third port were subtracted from the second port, with only clutter present, the subtraction process would yield a null. Therefore, as indicated in FIG. 1A-FIG. 3A, the clutter ground returns for the illustrated four transmitted pulses per antenna, in all three ports, look exactly alike in all corresponding range Doppler bins.
Referring to FIG. 1B, the return signals are vectorally represented for the case of a first mover in the absence of clutter, before and after spectral processing (FFT). As indicated, the signals from the first mover at the first port have the same spectral lines as clutter at that port. To view this another way, since the first mover occupies the same range Doppler bin as the clutter in port 1, the ground returns appear identical. Considering FIGS. 2A and 2B, return signals from the first mover at the second port are phase shifted by 90.degree. per pulse when compared with the return signals of clutter at port 2. The first mover in port 2 is phase shifted with respect to the first mover in port 1 as is apparent from the returns of FIGS. 1B and 2B. It is this phase difference as detected by adjacent ports of an antenna, in response to a moving target, which is employed to determine relative radial velocity of the first mover. Inasmuch as the clutter is considered, it has no phase shift between ports. Similarly, in port 3 the first mover has a phase shift relative to the first mover in port 2, in the same Doppler bin, as is apparent from the vector diagrams of FIGS. 2B and 3B. FIGS. 1C, 2C and 3C illustrate corresponding vector diagrams for a second mover which has a different radial velocity.
In FIGS. 1D, 2D and 3D the resultant of the first mover ground return plus the clutter return is vectorally represented by respective vectors M and C. Similarly, the cumulative vector effect of the second mover and clutter is indicated in FIGS. 1E, 2E and 3E. As will be observed in FIGS. 2D, 2E, 3D and 3E, phase shift in the second and third port signals occurs for the first and second mover vectors relative to the clutter vector due to the motion of the mover. In other words, as an antenna platform moves relative to a target, the second port will see a displaced target when that port occupies the position in space that the first port occupied an instant previously. Similarly, when the third port occupies the position in space which the second port occupied an instant previously, it will see the target displaced from the position it previously had, all of which results in the phase shift of the mover vector relative to the clutter vector.
In FIG. 4 the vector diagram is illustrated for the determination of relative radial velocity of a first mover. The vector diagram in FIG. 4 indicates each of the resultant vectors from ports 1-3 previously explained in connection with FIGS. 1D, 2D and 3D. These are respectively indicated in FIG. 4 as the vectors for ports 1, 2 and 3. Each of these vectors is not drawn to the same scale as FIGS. 1D, 2D and 3D but rather are drawn to exaggerate the differences of the resultant vectors at the ports so that phase determination can be more clearly indicated. Of crucial significance in the three-port system is the determination of the angle .phi. which is the phase difference between the vectors (port 1-port 2) and (port 2-port 3). The indicated phase difference of 90.degree. is proportional to the relative radial velocity of the first mover target which generated the returns at the three ports.
The true azimuth position of the target also depends upon this phase angle. In the event that the boresight of the antenna is coincident with the true azimuth position, the phase angle will be 90.degree. as indicated in FIG. 4. Accordingly, the first mover target discussed herein lies along the boresight of the antenna which generated the first mover returns at ports 1, 2 and 3.
In an implementation of the three-port system as shown in FIG. 6, the signals at ports 1, 2, and 3 undergo separate spectral processing employing the mentioned Fast Fourier Transform (FFT) at blocks 10, 12 and 14. Vector subtraction of port 1-port 2 occurs in vector subtractor 16 while the vector subtraction for port 2-port 3 occurs in vector subtractor 18. A phase processor 24 determines the phase angle from the subtracted vector inputs at 20, 22. The output from phase processor 24 undergoes computing at 26 to determine the relative radial velocity and azimuth position by computations well known in the art.
In order to better illustrate the technique of the three-port system, a second mover or target will now be discussed, this mover not being at the boresight of the antenna as was the case with the first mover. FIGS. 1C, 2C and 3C illustrate the four-pulse-return vector diagram of the second mover only, in the absence of clutter, before and after spectral processing by means of Fast Fourier Transforms. The resultant vector diagrams of the second mover plus clutter, after processing, is respectively indicated for each port in FIGS. 1E, 2E and 3E. These latter-mentioned resultant vectors are employed in the vector diagram of FIG. 5 where the vectors from ports 1, 2 and 3, for the second mover, are illustrated with the second mover resultant vectors for (port 1-port 2) and (port 2-port 3). As in the case of the first mover, FIG. 6 accomplishes the latter-mentioned vector subtractions; and phase detector 24 determines the phase angle illustrated in FIG. 5 which again is determinative of target relative radial velocity and true azimuth position. In the situation shown in FIG. 5, the angle is no longer 90.degree. which would be in line with boresight, as was the case of the first mover as shown in FIG. 4. For the second mover shown in FIG. 5, the phase angle indicates a true azimuth position which is off boresight and represents a relative radial target velocity different from that of the first mover (FIG. 4).
The moving targets are detected in ports 1, 2 and 3. They are detected in a Doppler bin proportional to their true azimuth position plus their relative radial velocity. The phase shift between port 1-port 2 and port 2-port 3 is equivalent to a Doppler bin that is matched to the phase shift between these vectors. This is proportional to the relative radial velocity.
The true target azimuth position is proportional to the phase difference due to its azimuth position. This is an equivalent Doppler bin. The true azimuth position is proportional to the Doppler bin of the detected target in port 1, 2, 3 minus the equivalent Doppler bin of the relative radial velocity. The result of the three-port system is the detection of a moving target and measurement by prior art techniques of various target parameters such as relative radial velocity, true azimuth position and even amplitude and range.
In order to gain a better appreciation between the various Doppler parameters and measured characteristics of a moving target, the overall relationship of velocity quantities may be expressed as: EQU V.sub.DT =V.sub.D +V.sub.DA
where V.sub.D equals Doppler due to target relative radial velocity of target to radar antenna;
V.sub.DA equals Doppler due to relative radial velocity due to a radar return existing at an angle relative to the boresight of an antenna; EQU V.sub.DT equals .DELTA..phi./.DELTA.t.
The vector subtraction previously discussed yields the value of .DELTA..phi. and V.sub.DT is the total Doppler detected by the spectral processor 10 as indicated in FIG. 6. The original equation may then be transformed as follows: EQU V.sub.DA =V.sub.DT -V.sub.D.
Inasmuch as the quantities on the right side of the equation are known, the quantity V.sub.DA may be solved. In order to further solve for azimuth position, it is necessary to consider the expression: ##EQU1##
Having solved for V.sub.DA and being able to obtain the velocity of the aircraft (V) which carries the radar platform, further knowing the wavelength of the radar transmitted frequency, it is a straightforward matter to compute .theta. which is the angular deviation of the target from the boresight of the antenna indicated at reference numeral 30 in FIG. 6.